Lithium is intercalated into many host materials. These materials include manganese oxides.
Sol-gel processing has become a common method to prepare macromolecular inorganic network materials via hydrolysis and condensation reactions that start from molecular precursors such as organometallic compounds or inorganic salts.
Recent reports have described the increasing worldwide research and development studies of rechargeable lithium (and lithium-ion) batteries that are currently underway.
Idoka, et al., Science 276, 1395 (1997) focused attention on tin-based amorphous oxide, while Sato, et al., Science 264, 556 (1994) described intercalation into disordered carbons, both of which serve as negative electrodes in lithium-ion cells. The reports have cited the importance of high energy cells for use in portable electronic devices and in electric vehicles. In the latter, the incentive is to lower pollution and the toxic load on the environment.
The positive counter-electrode (cathode) is also of importance, and is usually an intercalation material as well in which the chemical potential of lithium is lower than in pure lithium by several electron volts. Out of the large number of candidate cathode materials, the selection for specific applications is driven by considerations of cost, toxicity, and performance. In addition, the driving range is of major concern for electric vehicles that are powered by rechargeable batteries. Due to their low cost and low toxicity, manganese oxide materials have emerged as important alternatives to the present high energy cathodes based on lithiated cobalt and nickel oxides. This application focuses on synthesis and properties of nanoporous amorphous manganese dioxide, and its various modifications, as a reversible intercalation host for lithium. The amorphous materials of this invention do not suffer from the irreversible phase changes that are characteristic of the crystalline hosts and which limit their performance. The intercalation capacity and specific energy of the amorphous material exceed that of any crystalline manganese oxide, reported either as spinels, see Thackeray, M. M., David W. I. F., Bruce, P. G., and Goodenough, J. B., Materials Res. Bull. 18 451-472 (1983); Tarascon, J. M., Guyomard, D., Electrochimica Acta 38, 1221-1231 (1993); Pistoia, G., and Wang, G., Solid State Ionics 66, 135-142 (1993), or as layered materials by Armstrong, A. R., and Bruce, P. G., Nature 381, 499 (1996). Another amorphous material, manganese oxyiodide, has been reported very recently by Kim, J., and Manthiram, A., Nature 390, 265 (1997), and comparisons with the present material will be discussed further below.
Manganese oxides are among the most attractive cathode candidates for lithium batteries. Among the advantages they offer are low cost and relative non-toxicity, in addition to superior electrochemical properties such as high voltages. The most extensively studied form of manganese oxide cathodes is LiMn2O4 of spinel structure. Up to ca. 0.5 moles of lithium per mole of Mn can be intercalated reversibly into this cathode either in the 4 V region (x=0 to 1, LixMn2O4) or in the 3 V region (x=1 to 2, LixMn2O4). Recently another form of lithium manganese oxide, LiMnO2 of layered structure, was reported by Armstrong, A. R. et al, Nature 381, 499 (1996). Up to ca. 1 mole of lithium per mole manganese can be electrochemically extracted out of this material during first charge (x=1 to 0, LixMnO2); however, the charge/discharge capacity of subsequent cycles is less than 50% of that of the first cycle. It has been subsequently reported that lithium extraction and reinsertion into this material is not a reversible intercalation reaction and that the material is converted from the layered structure to the spinel structure upon cycling, see Vitins, G.; West, K.; J. Electrochem Soc., 144,2587 (1997).
Various methods for preparation of amorphous manganese oxides are known. For example U.S. Pat. No. 5,674,644 assigned to General Motors Corporation is directed to a xe2x80x9cManganese Oxide Electrode and Methodxe2x80x9d, which is specifically directed to a lithium ion cell.
Since this invention and the GM patent are both concerned with nominally xe2x80x9camorphous Manganese Oxidexe2x80x9d a comparison of the GM patent and this invention seems warranted. The important differences between the two are:
1. Although the materials covered by the GM patent (hereafter referred to as the GM materials) and the materials of this invention might both be referred to as xe2x80x9camorphous manganese oxidexe2x80x9d, they are very different materials with different chemical compositions and for different uses.
2. The chemical compositions of the materials of this invention are different from those of the GM materials. The GM materials contain a large amount of lithium or sodium, which is evident from FIG. 6 of the patent. The FIGURE, which is characteristic charge-discharge curves of the GM materials, shows the materials are charged first. That means they contain a large amount of lithium (or possibly sodium) and they are for use as the positive electrode for lithium ion batteries, as claimed throughout the patent. The materials of this invention contain no lithium and only a tiny amount of sodium.
3. The materials of this invention possess much higher charge capacity (milliampere-hour per gram (mAh/g) than the GM materials. The highest capacity among the GM materials is about 240 mAh/g, while the highest capacity of the materials of this invention is 436 mAh/g.
4. The most preferred among the GM materials, including the one that gives the highest capacity, contain electronically conducting polymers, such as polyaniline, mixed with manganese oxide, while the materials of this invention contain no conducting polymers whatsoever.
5. The method for synthesizing the materials of this invention is very different from the method for synthesizing the GM materials as described in the patent, although they can both be referred to as a xe2x80x9csol-gel method.xe2x80x9d xe2x80x9cSol-gel synthesis (or method)xe2x80x9d is very broad and entails different ways of synthesis (like the term xe2x80x9csolution synthesisxe2x80x9d). Specifically, the GM method involves mixing a solution containing manganese of high valence with a solution containing manganese of low valence, while the method of this invention involves mixing a solution containing manganese of high valence with a solution containing an organic reducing agent (no manganese). Further, the synthesis for this invention involves treatment with an acid such as sulfuric acid to induce a disproportionation reaction, while the GM method involves no such treatment. Still further, the method of this invention involves ultra sonication, while the GM method does not. There are still other differences between the two methods, which need not be described here.
6. In synthesizing the materials of this invention, the synthesis solutions are not heated at all. The entire synthesis process is carried out at room temperature. The synthesized materials in some cases may be heated to around 100xc2x0 C. before use. In the GM method, the synthesis solution is heated to 80xc2x0 C. for the synthesis process and the synthesized material is heated to 180xc2x0 C. before use.
7. The fact that the materials of this invention have different chemical compositions, much higher charge capacities, and are synthesized by a different method (and also most likely have a different crystal nano- and micro-structure), among other different factors, substantiate the difference between the materials of this invention and those of the GM patent.
8. The terms xe2x80x9camorphous manganese oxide (or dioxide)xe2x80x9d and xe2x80x9csol-gel methodxe2x80x9d are generic terms which do not describe specific technical content. There can be many different kinds of materials under the generic term xe2x80x9camorphous manganese oxide (or dioxide)xe2x80x9d with different local atomic arrangements and chemical compositions (not all amorphous structures are the same. They are all referred to as xe2x80x9camorphousxe2x80x9d only in that X-ray powder diffraction cannot tell a difference among them, but more detailed structural analysis by other techniques will reveal the differences among different xe2x80x9camorphousxe2x80x9d structures. Not all xe2x80x9ccrystallinexe2x80x9d structures are the same. Differences among xe2x80x9ccrystallinexe2x80x9d structures can be revealed by the X-ray powder diffraction technique, while the differences among xe2x80x9camorphousxe2x80x9d structures cannot be revealed by this technique, but can be revealed by other techniques). Similarly, there can be many different routes of synthesis involving different starting materials (precursors) and processes under the generic term xe2x80x9csol-gel methodxe2x80x9d.
9. In addition to the GM patent, two other patents cover xe2x80x9camorphous manganese oxidesxe2x80x9d and their use in lithium batteries (U.S. Pat. No. 5,62,329 and U.S. Pat. No. 5,601,952). We also found two Japanese patents JP 01131029 assigned to Nippon Telegraph and Telephone Public Corp. and JP 0167896 assigned to Sony Electric Co., Ltd., that cover xe2x80x9camorphous manganese oxidesxe2x80x9d and their use in lithium batteries. This suggests that an issued patent on one xe2x80x9camorphous manganese oxidexe2x80x9d and its use in lithium batteries does not exclude the patentability of an invention on another xe2x80x9camorphous manganese oxidexe2x80x9d and its use in lithium batteries.
Nanoporous amorphous manganese dioxide (a-MnO2) has been synthesized via a room-temperature sol-gel route. The material is a stable intercalation host for lithium and the intercalation capacity is greater than 1.6 moles of Li per mole of MnO2. The host remains amorphous in the entire intercalation range and the insertion process is reversible. When used as an intercalation cathode for lithium batteries, the material yields a charge capacity at the level of 436 mAh/g, and stores energy at the level of 1056 mWh/g. The former figure of merit represents an improvement over crystalline manganese oxide materials by a factor of about 4, and the latter by factors of 2 to 3. Various modifications of the material, such as slightly heated or polyvalent cation doped forms, retain the high capacity and energy of the original material and exhibit improved cycling performance. The above materials can be readily lithiated for use as cathodes for lithium ion batteries as well.
Fumaric Acid Reduction Method
The fumaric acid reduction method is based in part on J. Electrochem. Soc. 143:5, May 1996, over which the present method distinguishes. The stated article is directed to preparation of materials for use in alkaline Zn batteries, and is incorporated herein by reference. The entire fumaric acid reduction method described hereinbelow is performed at room temperature to ensure a product having an amorphous structure. Note that the methods of the invention may employ any reducing agent that would function as the fumaric acid disodium salt since the same results could be achieved by using well known substitute reducing agents. Likewise, although sulfuric acid is referred to in the method, other acids can be employed as is well known in the art.
A first solution of sodium permanganate solution, 300 ml of 0.25 M NaMnO4 is prepared. While the first solution is being vigorously stirred, a second solution, 75 ml of 0.300 M fumaric acid disodium salt C2H2O4Na2 is gradually added. The resulting mixture is stirred vigorously overnight to ensure completion of the reduction reaction. Following ultrasonification for six hours at room temperature in a constant-flow water bath which prevents temperature increases, 7.5 ml of 2.50M H2SO4 is added. The resulting mixture is then stirred vigorously overnight, then the precipitates are washed with distilled water. The resulting manganese oxide has manganese of mean oxidation state close to four as determined by a redox titration method using ferrous ammonium sulfate Handbook of Manganese Dioxides Battery Grade, International Battery Materials Association (1989). The resulting material is mixed with a carbon powder and a binder, and pellets are made therefrom.
Electrochemical measurements are performed on the resulting pellet(s) which is discharged and cycled in a nonaqueous Li solution such as LiClO4 solution in a suitable organic solvent such as propylene carbonate.